1. Field of the Invention
The present invention relates to centrifuge instruments and in particular, to a centrifuge instrument operable in either an evacuated or a non-evacuated environment.
2. Description of the Prior Art
Current centrifuge instruments can be divided into two basic types--those that operate with the chamber at atmospheric pressure and those that operate with the chamber evacuated to a pressure lower than atmospheric. In general, lower centrifugal force applications are performed in a centrifuge where the chamber is at atmospheric pressure. An example of such an instrument is the RC-5C Centrifuge marketed and sold by the Medical Products Department of E. I. du Pont de Nemours and Company, Inc. This type of centrifuge is inherently simpler, less expensive and more reliable than a centrifuge whose chamber is evacuated. There is no vacuum pump nor are plural seals necessary to isolate the chamber from atmospheric conditions. This means fewer parts, less strict machining tolerances and less maintenance concerns. Additionally, the rotors, tubes and bottles used in this type of centrifuge are also inherently simpler and less expensive in that no seals are required to isolate the sample in its container from a vacuum environment. For all these reasons, operation at atmospheric pressure is generally considered to be the preferred method of operation.
However, operation at atmospheric pressure has some limitations. A rotating body in a non-evacuated environment creates windage. This windage has two detrimental effects on centrifuge performance. First, windage opposes the drive torque and as such acts to limit the maximum angular velocity of a rotor. Eventually, a point is reached where the windage (and other viscous losses such as bearing losses) equals the drive torque output from the motive source. At this point none of the applied torque is used to accelerate the rotor thereby limiting the angular velocity of the rotor. Limiting the angular velocity of the rotor also limits the centrifugal force to which the sample can be exposed as relative centrifugal force (RCF) is a function of its radius and the square of the angular velocity of the rotor. Second, windage creates heat that tends to elevate the temperature of the sample. While, in general, centrifuges have a cooling system to control sample temperature, this cooling system has a predetermined capacity. The heat generated by windage must never exceed the cooling capacity of the centrifuge temperature control system.
In order to reduce windage and these limitations imposed by the same other centrifuge instruments operate with the chamber evacuated. An example of such an instrument is the OTD Ultracentrifuge instrument marketed and sold by the Medical Products Department of E. I. du Pont de Nemours and Company, Inc. These instruments are generally used only for high centrifugal force applications for reasons generally opposite to those described above as advantages for the non-evacuated chamber system. The chamber is always evacuated during operation of the centrifuge instrument. Evacuation creates stresses on the framework of the centrifuge and the seals which isolate the chamber from atmospheric conditions. These stresses create the need for periodic maintenance especially for the seals and the vacuum pump.
It is believed advantageous to provide a centrifuge that is operable in an evacuated environment in order to create high centrifugal forces and also operable in a non-evacuated environment in order to use the less expensive, less complex rotor, tube and bottle systems. It is also believed to be advantageous to minimize the use of the evacuation to only those applications specifically requiring the same in order to minimize the stress placed on the seals and the maintenance associated therewith. Further, it is believed to be advantageous to provide an instrument that can automatically control the pressure in the chamber based on the identity of the rotor loaded onto the drive and the requested run parameters.